Coating surfaces with polymers is usually done by evaporating the solvent from a solution of the polymer on a surface or by thermally fusing a powder coating which has been applied by some method such as electrostatic spraying. The former technique requires relatively large volumes of solvent, because the solids content of solutions are usually 10-50%. Fusing powder coatings is receiving wider acceptance, because no solvent is required and very little polymer is lost in the process.
Polymers which have limited solubilities or whose solutions have unusually high viscosities at relatively low solids content are not practically suited for deposition from solutions. In addition, polymers which soften at high temperatures or cross-link rather than fuse or decompose at temperatures close to their softening points cannot be used, or find limited use in techniques whereby a dry powder coating is thermally fused into a continuous film.
For certain commonly used low melting thermoplastics such as polyvinylchloride and polyacrylates, powder coalescing techniques have been described, e.g. U.S. Pat. No. 3,676,171, and U.S. Pat. No. 3,676,172. Nevertheless, the techniques described in those patents are only applicable to a certain class of thermoplastics which soften at relatively low temperatures. Furthermore, the solvents used are not especially persistent and can be removed with considerable ease as opposed to the solvents used in the technique of the invention. In addition thereto, the conditions of use of the coalesing techniques of the art are different than that which will be described herein.